Executing Byte Array in Go

First, you don't need to (currently) use mmap at all, since go memory is executable. If you do need mmap, you can use anonymous memory and forgo the temp file:

b, e := syscall.Mmap(0, 0, len(shellcode), syscall.PROT_READ|syscall.PROT_WRITE|syscall.PROT_EXEC, syscall.MAP_ANON)
copy(b, shellcode)

Otherwise, you can try to use shellcode directly, since it's already backed by a contiguous array.

As for converting the bytes in shellcode to a function, the analog from C would look like:

f := *(*func() int)(unsafe.Pointer(&d[0]))

which creates a function value named f which can then be called like a normal function.

If the shellcode isn't specifically crafted fro Go, and you need to call it from a C stack, it would be easier to do it directly in C using cgo.

/*
call(char *code) {
    int (*ret)() = (int(*)())code;
    ret();
}
*/
import "C"

func main() {
    ...
    // at your call site, you can send the shellcode directly to the C 
    // function by converting it to a pointer of the correct type.
    C.call((*C.char)(unsafe.Pointer(&shellcode[0])))

There are multiple ways to do it, though I have not tested them myself.

You may want to have a look at https://github.com/nelhage/gojit/blob/master/jit.go It implements a solution based on cgo (which is safer, but slower), and a solution based on a direct call. Look at all the build related functions.